Be aware that blood pressure acts a bit like water in a plumbing system, and so it is very much affected by the relative elevation of the point it is measured. Amazing how many GPs and nurses don’t seem to know this.
To illustrate, sit with upright back at a table and put the cuff on your arm. Now, raise the cuffed arm aloft, hand resting on back of your head, and take a reading. Chances are, both systollic and diadtollic will be very low. Now do it again, but with other arm draped down to the floor, BP should be even lower.
Now go to the other extreme, drape your cuffed arm down to your ankles, your reading will be very high. Raise the other arm to the ceiling and try again, it will be even higher.
In between these extremes, in more normal postures, lies a continuous variation of measurements. I used to play a game, getting people to “request” that I demonstrate a given BP. By subtly and almost unnoticeably rearranging my limbs whilst sitting in an armchair, whilst still maintain a broadly neutral posture, I was able to satisfy quite a wide range of requests.
I am no expert in medical significance of BP readings, just pointing out the limitations of measurement technique. I do believe the important thing is the cuff should be at heart level for the correct reading, but how many of us actually know exactly where the heart is? And even harder to judge, when lying down.